The majority of radio receivers utilise a superheterodyne type receiver which shifts the carrier frequency of a selected radio channel from an initial high frequency to a lower fixed intermediate frequency (IF) prior to demodulating the information signal. The superheterodyne receiver performs band-pass filtering at the IF frequency so that the bandwidth of the filter can be relatively small whilst still allowing for receipt of a large number of channels from which a desired channel can be chosen. The dynamic range of the superheterodyne receiver is therefore considerable.
An alternative to the superheterodyne receiver is the homodyne or direct conversion receiver which comprises a tunable local oscillator (LO) which is tuned to the carrier frequency of the radio signal to be received. The LO signal is mixed with the radio signal to directly down-convert the radio signal into a baseband signal. Due to the relative simplicity of the direct conversion receiver, there has long been interest in using such receivers in applications where cost and size are of critical importance. One such application is that of mobile communication devices. However, the limited dynamic range provided by direct conversion receivers has in practice restricted their use. The key factor responsible for this limitation is the spurious amplitude modulation (AM) and DC components generated by the mixer in the demodulated signal. As is illustrated in FIG. 1, this interference arises from RF signals leaking from the RF input 1 of the mixer 2 to the local oscillator input 3 which in turn gives rise to even ordered products in the output 4 of the mixer 2. For example, with a signal sinx, leakage results in a product (sinx).sup.2 =1 +sin2x where the first term contributes a DC component.
In the case of digital data reception, the receiver decides upon whether or not a received digital data sample is a 0 or a 1 on the basis of the DC voltage level of the demodulated signal. Thus, any DC offset caused by RF signals coupling to the local oscillator input of the mixer (or LO signal leaking into the RF input) can result in the receiver making a wrong decision as to the state of the received signal. In direct conversion receivers, it is necessary to bandpass filter the input signal over a relatively narrow bandwidth in order to minimise cross-channel interference. The dynamic range of a direct conversion receiver (or the number of channels which it can receive) is therefore relatively small compared to that of a superheterodyne receiver.
A number of potential solutions to this problem have been proposed. These tend to rely upon the use of DC filters. However, when a narrow band filter is used, the settling time becomes long such that the filter cannot react to quick changes of power. On the other hand, with a wideband filter it is possible to achieve a short settling time but at cost of filtering out a substantial part of the wanted signal.
U.S. Pat. No. 5,212,826 describes a method of compensating a wanted signal for DC offset by blocking the received RF signal from entering the receiver during periods when the wanted signal is not transmitted. The DC offset during this period is measured and a resulting constant correction factor is calculated and fed into a correction circuit which processes the RF signal during the subsequent reception period. However, because the correction factor is constant over a continuous reception period, variations in the DC level during this period are not compensated for.
There is described in U.S. Pat. No. 5,179,730 a direct conversion receiver suitable for demodulating quadrature phase shift modulated (QPSM) signals. Conventional QPSM receivers split a received RF signal into two signal components. These components are applied to respective mixers, with the first mixer mixing the received signal with a LO signal which is 90.degree. out of phase with the LO signal applied to the second mixer. U.S. Pat. No. 5,179,730 addresses the problem of mismatching occurring between the two mixers and the LO signals and which results in phase and amplitude errors in the demodulated baseband signals. U.S. Pat. No. 5,179,730 proposes providing a third mixer which mixes the input signal with a third LO signal which is out of phase with both of the LO signals applied to the first and second mixers. The outputs of the mixers are combined to produce a correction signal which is used to compensate phase and amplitude errors in the in-phase and quadrature phase signals. This document does not address the problem of spurious amplitude modulation (AM) and DC components generated at the mixer. Indeed, if such components are present, the efficiency of the mixer mismatch correction will be reduced.